Conventionally, coating a part is a two-stage or three-stage process, depending on whether the part to be coated is new or was already in service. If the part is new, in the first stage, the surface to be coated is prepped using grit blasting. In the second stage, following prepping, the coating is applied to the surface. If the part was already in service, then the three-stage process consists of stripping the existing coating, followed by prepping, then applying the new coating. Many different coating and plating technologies are known. What follows is an overview of some of the main technologies currently in use today.
Chrome Plating
Chrome plate has been the mainstay for applications with intense stress, such as the landing gear on aircraft. Other applications include extreme wear conditions, like the hydraulic shafts on heavy earth moving vehicles. It offers the following properties:
(i) Hardness: At 65 to 70-Rc (850-1000-Hv), hard chrome plate is harder than most industrial abrasives. Combined with inherent toughness, it can withstand high stress contact.
(ii) Substrate adhesion: With adhesion greater than 10-kpsi, hard chrome coatings can endure large stresses without detaching from the substrate.
(iii) Chrome can be applied on a wide variety of substrates.
(iv) Chrome responsive finish: responds well to grinding and polishing.
(v) Chrome may be applied to substrates having different geometries: because it is applied via plating, chrome can provide a high-quality coating on most complex geometries, unlike several of the alternative technologies.
(vi) Corrosion resistant: the coating is stable in the presence of many corrosive substances (organic acids, etc).
(vii) Wear resistant in metal-on-metal sliding: Chrome plate coatings are nearly 100 times as resistant to wear in situations with two dry metal surfaces sliding against each other, compared to nitrided steels.
(viii) Abrasion resistant: Even under high contact stresses, chrome plate exhibits a low wear rate against abrasive materials (>100 times than hardened steel).
(ix) Produces a low friction surface: Offers a smooth low friction surface.
(x) Compatible with lubricants: Cracked chrome, with small cracks in the surface, holds lubricants enhancing wear life and lubricity in demanding applications.
The electrocoating itself is performed by charging a low-voltage direct current through the bath. The part to be plated is usually the cathode, and the anode is typically a bar of lead-antimony or lead-tin alloy.
For hard chromium plating, the plating solution contains ions of hexavalent chromium from chromic acid, as well as sulfuric acid to improve the bath's conductivity. As the electricity is supplied to the solution, electrolytic decomposition of the water in the bath releases hydrogen gas at the cathode and oxygen at the anode. The result of the plating process is the deposition of metallic chromium on the substrate. Typical process parameters for hard chrome plating are: chromic acid: 250-g/liter (or 400 for concentrated), sulfuric acid: 2.5-g/liter (or 4), temperature: 55° C. (or 50°) and deposition rate: 25-Φm/hr (or 13).
Toxic emissions from chrome plating arise from the plating step itself and from the chromic acid anodizing step. In the plating step, mists of the electrolyte (mostly chromic acid) are generated by the rise of hydrogen and oxygen bubbles through the bath. The emission rates are affected by the bath temperature, concentration of chemicals in the bath, the surface area undergoing plating, and the plating current. As hard coating consumes higher current, it generates more chromic acid mist. The emissions are controlled by using hoods and more recently by employing scrubbers, electrostatic precipitators and activated carbon filters.
Hexavalent chromium (CrO3) has long been known to be dangerous for workers at chrome plating facilities, causing many health problems (irritated nasal septum, dermatitis, etc). Another difficulty with chrome plating is elemental chrome dust resulting from grinding, which has been shown to be carcinogenic. Consequently, OSHA (Occupational Safety and Health Administration) has proposed to lower the permissible exposure limit (PEL) for workers from 52-Φg/m3 to 1-Φg/m3 as an 8-hr time weighted average, which will significantly increase the cost of chrome plating (may apply to nickel plating also).
There are a number of disadvantages of chrome plating.
First, toxic emissions in the chrome plating, which arise from the plating step itself, and from the chromic acid anodizing step. In the plating step, mists of the electrolyte (mostly chromic acid) are generated by the rise of hydrogen and oxygen bubbles through the bath. The emission rates are affected by the bath temperature, concentration of chemicals in the bath, the surface area undergoing plating, and the plating current. As hard coating consumes higher current, it generates more chromic acid mist. The emissions are controlled by using hoods and more recently by employing scrubbers, electrostatic precipitators and activated carbon filters.
Second, hexavalent chromium (CrO3) has long been known to be dangerous for workers at chrome plating facilities, causing many health problems (irritated nasal septum, dermatitis, etc). Another difficulty with chrome plating is elemental chrome dust resulting from grinding, which has been shown to be carcinogenic. Consequently, OSHA (Occupational Safety and Health Administration) has proposed to lower the permissible exposure limit (PEL) for workers from 52-Φg/m3 to 1-Φg/m3 as an 8-hr time weighted average, which will significantly increase the cost of chrome plating (which may apply to nickel plating also).
Third, chrome plate offers properties that are generally good, but hardly ever excellent. The material offers limited hardness and corrosion resistance. It suffers from pitting, spalling and other failures under stressful conditions. Users became generally aware that for specific applications, there may be substitutes that perform better than chrome plate at a comparable or even lower cost. For a number of specific applications, alternative technologies have already proven their value. In the long run, chrome dominance in many applications has diminished.
Fourth, hard chrome has a high intrinsic stress leading to a significant fatigue drawback. Fatigue issues in aircraft are serious concerns for structural components and critical hydraulics. Consequently, such parts are routinely treated with shot peening to eliminate stress.
Fifth, chrome plating also causes hydrogen embrittlement in high strength steel substrates used in landing gears, etc. For this reason, these parts must be heat treated after plating, adding to the overall cost and processing time. It also suffers pitting in humid and corrosive environments. Masking also is difficult in chrome plating.
Sixth, hard chrome's intrinsic brittleness leads to micro-cracking, which are unsuitable when corrosion resistance is needed, requiring nickel underlayers. Also, the rate of deposition is slow (4 to 8-microns/hr compared to 30-microns for nickel plating). Also, the coating process lacks uniformity, requiring follow-up machining to dimensional tolerances.
Seventh, aircraft parts that are chrome plated are exposed to severe wear and corrosion stresses during service. Because these parts also tend to be expensive, they demand several refurbishments during their lives. Chrome plating is also used to restore dimensional tolerances of parts which were not originally chrome plated. Chrome plating is used both in commercial and military aircrafts, which are done in several ways.
Hydraulic actuators and landing gears are important applications of chrome plate in aircraft. These are complex and expensive and are generally made from high strength steels. Several actuators are used to raise, lower and manipulate the landing gear. Most aircraft will have anywhere from 25 to 100 actuators.
Landing gears are large components that see service for brief periods of time but must bear enormous stress on landing and, occasionally, takeoff. A landing gear inner cylinder can be three to eight feet in length and two to six feet in diameter. The inner cylinder is often a single machined part that includes the piston and wheel bearing journals. The piston is plated over most of its length, and the journals are plated in the bearing journal areas. The chrome plate is typically 0.003-in thick. All landing gears are chrome plated for resistance to wear and corrosion. Aircraft must have at least three landing gears, while some of the larger airlines (e.g. Boeing 747) may have four or five.
Alternatives to Chrome Plating
Trivalent Chromium is much less hazardous than hexavalent chromium but will also allow electroplating. Process improvements and technological advances have changed trivalent chromium from inferior substitute to a serious contender with properties that rival those of hexavalent. It also allows 100% recycling. However, it remains expensive, and is less tolerant to impurities. It is profusely used in the auto industry (where there is much less of a regulatory burden). In terms of environmental protection, regulations are the same as hexavalent.
Thermal Spray
Thermal spray refers to at least 14 different types of coating processes that use thermal and kinetic energy to accelerate particles from powder or wire onto a substrate. The forms of thermal spray differ widely in how they use that thermal and kinetic energy, which in turn produces significant variation in the coatings supplied.
The various types of thermal spray are organized under the following families: flame spray, plasma spray, and electric arc. Each thermal spray technique involves basically the same process. First, raw material is fed into a heat source, i.e., thermal spray gun, where it is melted and then blown onto the substrate in the form of molten droplets. The droplets impact the surface of the substrate where they deform around the features in the roughened surface and solidify in a disk shape.
Thermal spray offers a number of advantages over competing coating methods. While some thermal spray technologies are limited in their choice of raw materials, a virtually unlimited range of materials can be accommodated by at least one method. Thermal spray allows a fast deposition rate, ability to deposit on highly heat sensitive surfaces, use of portable equipment and is often quite economical. Coatings are mechanically bonded to the surface, making chemical compatibility less of an issue compared with other coating methods. Thermal spray can be used to deposit coatings of plastics, metals, carbides, and ceramic coatings as thick as 0.25-in (6.35 mm). An added benefit is that no wet chemicals are used in the process, thus minimizing any environmental impact. While water is used to cool thermal spray equipment during operation, it never comes into contact with potentially hazardous materials and so no treatment is necessary. Overspray material can be captured and recycled.
Plasma Spray
Rather than burning a fuel gas, plasma spray ionizes a gas such as nitrogen at very high temperatures to produce a plasma which melts powder particles and sprays a dense coating at a somewhat limited velocity. It uses the highest temperature heat source; the core of the plasma can reach temperatures as high as 16,000° C. Along with the high thermal energy, it produces high gas velocities, up to Mach 2. The high temperatures used in plasma spray enable the method to work with materials with very high melting points, like some ceramics, with good properties. The high particle velocity allows densities in the coatings of up to 98 percent. It uses relatively large powder particles, usually in excess of 45-Φ in diameter, although diameters as low as 15-Φ have been used. Because of lower particle velocities at high temperatures, bond strengths are lower and porosity higher compared to HVOF (High-Velocity-Oxygen-Fuel). It offers limited corrosion protection, especially compared to HVOF. Plasma coatings suffer from interconnecting porosity even at thicknesses of 30-Φ, which exposes the bare metal. It has proven itself an inexpensive method for building up a surface, adding metal on spots where the surface has been worn or damaged. It is extensively used for forming thermal spray barriers.
D-Gun
The D-Gun (developed by Praxair) process involves bringing gas, fuel (usually acetylene) and powder into a chamber, igniting the mixture and emitting it from a barrel. The process works by intermittent combustion, pulsing the ignition six times per second, unlike HVOF which is continuous and less expensive. There are a range of proprietary coatings available with D-gun, some of which may result in denser coatings than HVOF or more varying metallographic structures. It is more costly to run than conventional HVOF, because of the complexity and control requirements for pulsing the ignition. The motion of the part to be coated also has to be coordinated with the timing of the gun's firing, adding to the complexity and cost of the process. HVOF has become widely available.
HVOF coatings applied in wear applications provide superior performance in most situations compared to chrome. Compared to chrome, HVOF shows its strength in coating large parts. A plating tank for a large component could be very expensive to set up and maintain, but applying HVOF to a large surface requires no significant investment in terms of capacity, assuming the part fits in a standard cell. For small parts, chrome plating is better. Surface complexity of treated parts is also a factor. While chrome plating easily reaches the entire surface of a complex part, using HVOF is proportionately is more difficult. The inability of HVOF to be applied to some inner diameters and non-line of sight geometries, common to all thermal spray methods but more critical with HVOF, will continue to be a significant liability of the technique. Ultimately, the non-line of sight issue will place a fundamental limit on HVOF's market penetration.
Masking is more difficult in HVOF processing than it is in plating, with potential issues in surface transition areas. HVOF's chief disadvantage is its high up-front costs compared to other coating methods, including consumables and equipment. However, cost comparisons generally ignore long-term economic benefits such as faster TAT (turn-around-time), longer wear lifetime of parts, reduced hazardous wastes, etc.
On average, one HVOF cell is able to process a single landing gear in 40-minutes, with labor rates nearly the same as those for hard chrome plating. TAT for landing gear components is typically five days less than the average for chrome plated components. The advantage of HVOF, or any thermal spray methods, comes largely through the reduced process time. Total process hours for an HVOF application are as little as a fifth of electroplated hard chrome. The high cost of HVOF compared to plasma spray is somewhat mitigated by the finer powders used and denser coatings that result, because the surfaces typically need less finishing work.
Twin Wire Arc Spray
Twin Wire Arc Spray makes use of two wires of opposite polarity that are fed simultaneously and meet at a point, where arcing occurs and melts the wires. A blast of compressed air or inert gas atomizes the metal and forms a spray whose velocities often reach Mach 1. The method provides excellent deposition rates and are capable of depositing hundreds of pounds of material per hour. It is used primarily in relatively low-end corrosion protection applications, and in providing material buildup in refurbishing. It is also quite inexpensive compared to HVOF (quite portable and overall processing costs are about 10th of HVOF). Good method for chrome substitution (used profusely in petrochemical industry). However, the density of the coating is about 92% compared to about 98.5% for HVOF. Low density is the greatest drawback. Also, the choice of materials is limited.
Cold Spray Technique
Cold spray process uses high velocity rather than high temperature to produce coatings, and thereby avoid/minimize many deleterious high temperature reactions, which are characteristics of typical thermal spray coatings. Typical advantages of cold spray coatings include compressive rather than tensile stresses, wrought-like microstructure, near theoretical density, and free of oxides and other inclusions. Moreover, the footprint of the spray beam is very narrow yielding high-density particle beam, which results in high growth rate of coating thickness with a better control over the shape of the coating, without masking requirements. The basic principle of cold spray is quite simple. When a particle-laden gas jet impinges on a solid surface, three different phenomena occur, depending on the particle velocity, Vp. When Vp is low, the particles simply bounce (reflect) off the surface. When it reaches moderate values, they erode the surface. When it exceeds a critical value (which varies with particle and substrate materials—typically in the range of 500 to 900-m/s), particles plastically deform and adhere to the substrate, one another to form an overlay deposit, analogous to the thermal spray process.
The process uses a high-pressure, high-velocity gas jet to impart the velocity for the coating particles. The gas jet, preheated to compensate for the adiabatic cooling due to expansion, is expanded through a Laval nozzle to form a supersonic jet. Powder particles, transported by a carrier gas, are injected into this gas jet. Momentum transfer from the supersonic jet to the particles results in a high-velocity particle jet. These powder particles, on impact onto the substrate, plastically deform and form interlinking splats, resulting in a coating.
The gas delivery system supplies up to 170-m3/hr (100-scfm) of nitrogen and or helium at the pressures of 15 to 40-bars (215 to 280-psi). An electric gas heater heats the gas to a maximum of 923-° K (1,200-° F.). Powdered coating material, in the size range of 5 to 45-microns, is delivered by the powder hopper and transported by a carrier gas to the gun. The control console houses all the controls to meter the flow rates, etc.
Cold spray is a solid state process and hence produces coatings with many advantageous characteristics. Since high temperature is not involved, it is ideally suitable for spraying temperature sensitive materials such as nanophase and amorphous materials, oxygen-sensitive materials like aluminum, copper and titanium and phase sensitive materials such as carbide composites. Due to small size of the nozzle (10 to 15-mm2) and spray distance (5 to 24-mm), the spray beam is very small, typically around 5-mm in diameter, leading to good deposition. The process works similar to micro shot peening, and hence the coatings are produced with compressive stresses. Thus, ultra thick (i.e. 5 to 50-mm) coatings can be produced without adhesion failure. The high-energy low temperature formation of coating leads to a wrought-like microstructure with near theoretical density values.
Cold spray, owing to its principle of impact-fusion coating build-up, is limited to the deposition of ductile metals and alloys (Zn, Sn, Ag, Cu, Al, Ti, Nb, Mo, NiCr, Cu—Al, nickel alloys and MCrAlYs) and polymers, or blends >50% by volume of ductile materials with brittle metals or ceramics.
Major disadvantages of the cold spray process include the use of high gas flows, increased gas costs, especially in the case of helium, recycling would be needed. There are other problems such as abrasion of nozzles.
Powders
The dominant powder used as thermal barrier coating is yttria-stabilized zirconia (applied to gas turbine blades). Other powders are: WC, CrC, WC—Co, WC—Co—Cr (used as a chrome replacement material) and CrC—NiCr (used for protection against high temperatures of the order of 1,700° F., WC breaks down at about 800° F.).
Line of Sight (LOS) & Inner Diameter Issues
The inability of HVOF on NLOS geometries will ultimately limit the penetration of HVOF in aerospace to 80% in chrome replacement in aerospace, due to the fact that nearly 20% are NLOS, Plasma & HVOF gun extensions for inner diameters are commercially available but of limited usefulness. The coatings on the inner diameters are inferior. There are several problems in using HVOF equipment for small inner diameters, the limit at present being 2.5-in (63.5-mm).
Stripping and Grinding Issues
The bond strength, density and hardness that give thermal spray coatings their advantage also create their greatest difficulties in stripping and grinding (worse for HVOF). Grinding is usually performed to restore dimensional tolerances and to provide smooth finish. Surfaces coated with thermal spray & HVOF tend to be rough. Finishing requires not only an optimized process but also post-process grinding with a diamond wheel, which is significantly higher than the silicon carbide wheel used in chrome plating.
Stripping is a more significant problem. Unlike chrome, which undergoes less severe interaction with the substrate, HVOF involves molten metals at thousands of degrees applied to the surface, where they penetrate to form an alloy with the substrate and supply a molten surface to allow additional buildup. The consequence of the coating-substrate alloy formation is that some parent metal has to be sacrificed in removing the coating. Eventually a part that has been stripped may reach reduced dimensions where no more spray can be applied because it is no longer cost effective or no additional loss of substrate can be tolerated, and the part must be discarded. Stripping an HVOF coat may require leaving a part in a chemical stripping tank for a week, compared with a day for chrome. As a rule, the corrosion resistant alloys, such as WC—Co—Cr, CrNi and others, take longer and present more difficulties in stripping than WC—Co.
From the above, it is evident that there remains a need in the industry for more efficient stripping, prepping and coating techniques that do not give rise to the issues described above.